In recent years, a product display rack system has been proposed in which an RFID (Radio Frequency Identification) tag having a radio communication function is attached to a product, and by carrying out communications with the RFID tag, a product that has been taken from a desired display rack by a customer in the hand is detected so that information as to the degree of interest of the customer with the product is collected by using a computer. With respect to a document that describes the technique about this product display rack system, the following Japanese Patent Application Laid-Open Publication No. 2007-115152 is proposed. The product display rack system described in Japanese Patent Application Laid-Open Publication No. 2007-115152 is provided with: an intelligent rack including a product display rack on which products of a plurality of types to which RFID tags are attached are mounted; a plurality of antennas for detecting a tag serial number from the RFID tag of the corresponding product; and a pair of human detection sensors placed on the two ends of the product display rack; and a tag-product ID registration table in which the tag serial number and the product ID applied to each of the product kinds are stored in association with each other, and in this technique, by using the combination of the inputted tag serial number and the antenna that has detected the tag serial number, the fact that a specific product is taken from, or returned to a specific product display rack, is detected, and by using a time difference of the detections of the paired human detection sensors, the moving direction of a human body is also detected so that information relating to the degree of interest of the customer with the product is obtained.